You might have some defective memory modules, I recommended using a memory diagnostics tool called Memtest
The latest version of Memtest X86 is available here:
http://www.memtest.org/ Memtest X86 CD:
Download the newest ISO version (memtest86+-.iso.zip) to your PC.
-Extract the .zip file to a location that you know, like the Desktop or Downloads folder or next to the .zip file. You should end up with the file Memtest86+-1.70.iso
*Note* If you have WinRAR (or similar) installed, it will look like an unzippable archive with nothing inside. Do not open it with WinRAR.
-Go to your favorite CD burning software, and choose to burn from an image file. Simply point to the memtest.iso file and hit the "burn" button.
Eg: Nero>Disk Image or Saved Project>[select the memtest iso file]Ok>Next etc.
-Insert a blank CD into the drive and let it do its magic.
-Presto, you have a memtest bootable CD.
Simply leave the CD in the drive and boot from CD by pressing any key (if prompted) the test will run on its own and report any errors detected.